1. Field of the Invention
The present invention relates generally to hydraulic fracturing of oil wells, and particularly to a method for determining the magnitude of the least principle horizontal stress existing within a given formation.
2. Prior Art
After a well has been drilled to the desired depth of production, it is necessary to fracture the formation if production of commercially significant quantities of hydrocarbons are to be produced from the well. Fracturing procedures generally involve pumping a fracturing fluid to a given area of the formation under sufficiently high pressure so as to cause fractures to initiate in the formation. When fracturing operations are completed, these fractures extend radially away from the well bore and run in a direction perpendicular to the least principle horizontal stress existing within the field.
Many different techniques have been employed to determine the magnitude of the minimum principle stress within a formation. One of the techniques employed is known as an open hole microfrac test. The open hole microfrac test procedure is the subject of U.S. Pat. No. 4,529,036, which is hereby incorporated by reference. In that procedure, the magnitude of the least principle horizontal stress in the formation is determined by recording and interpreting pressure responses occurring during a series of pump shut in and flow back tests after fractures have been initiated in the formation. In particular, the magnitude of the minimum principle stress may be estimated by interpreting changes in the slope of a graph of the downhole pressure readings made during the microfrac procedure.
However, the information obtained during an open hole microfrac test was sometimes inaccurate due to the difficulty in recording and interpreting pressure responses during the fracturing procedure. Therefore, a need exists within the industry for a method that will allow accurate and reliable determination of the magnitude of the minimum principle horizontal stress existing within a formation. The invention disclosed and claimed herein satisfies this requirement.